Activation of Hematite Photoanodes for Solar Water Splitting: Effect of FTO Deformation

Annamalai, Alagappan,Subramanian, Arunprabaharan,Kang, Unseock,Park, Hyunwoong,Choi, Sun Hee,Jang, Jum Suk American Chemical Society 2015 The Journal of Physical Chemistry Part C Vol.119 No.7

<P>The sintering at 800 °C is found to induce the diffusion of Sn from the F-doped SnO<SUB>2</SUB> (FTO) into the hematite lattice, enhancing the photoelectrochemical cell (PEC) properties of the hematite photoanodes, but this diffusion also has detrimental effects on the conductivity of the FTO substrate. In the present research we examined the role of FTO deformation during the activation of hematite photoanodes synthesized on FTO substrates. The incorporation of Sn dopants from the FTO substrates in the hematite lattice was confirmed by X-ray photoelectron spectroscopy and was found to increase with sintering time. Further from the extended X-ray absorption fine structure analysis, it was found that the diffused Sn atoms affected the metal sites of the hematite lattice. Increased diffusion of Sn into the hematite lattice caused structural disordering of the FTO, but optimum sintering time compensated for the structural disordering and improved the ordering. Under high-temperature annealing at 800 °C, the FTO substrates underwent a stoichiometric change that directly affected their electrical conductivity; their resistivity was doubled after 20 min of sintering. Activation of hematite photoanodes by high-temperature sintering entails a kinetic competition between Sn dopant diffusion from the FTO substrate into the hematite and the resulting thermal deformation and conductivity loss in the FTO substrates.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2015/jpccck.2015.119.issue-7/jp512189c/production/images/medium/jp-2014-12189c_0013.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp512189c'>ACS Electronic Supporting Info</A></P>

Role of Graphene Oxide as a Sacrificial Interlayer for Enhanced Photoelectrochemical Water Oxidation of Hematite Nanorods

Annamalai, Alagappan,Kannan, Aravindaraj G.,Lee, Su Yong,Kim, Dong-Won,Choi, Sun Hee,Jang, Jum Suk American Chemical Society 2015 The Journal of Physical Chemistry Part C Vol.119 No.34

<P>Photoelectrochemical cells (PECs) with a structure of F-doped SnO<SUB>2</SUB> (FTO)/graphene oxide (GO)/hematite (α-Fe<SUB>2</SUB>O<SUB>3</SUB>) photoanode were fabricated, in which GO serves as a sacrificial underlayer. In contrast to low-temperature sintering carried out under a normal atmosphere, high-temperature sintering was carried out for the GO underlayer-based hematite photoanodes. The photocurrent density of the PECs with GO underlayers gradually increased as the spin speed of the FTO substrate increased. In particular, GO at a spin speed of 5000 rpm showed the highest photocurrent of 1.3 mA/cm<SUP>2</SUP>. The higher performance of the GO/α-Fe<SUB>2</SUB>O<SUB>3</SUB> photoanodes was attributed to the improved FTO/α-Fe<SUB>2</SUB>O<SUB>3</SUB> interface. When sintered at 800 °C for activation of the hematite (FTO/GO/α-Fe<SUB>2</SUB>O<SUB>3</SUB>) photoanodes, the GO layers before being decomposed act as localized hot zones at the FTO/α-Fe<SUB>2</SUB>O<SUB>3</SUB> interface. These localized hot zones play a very crucial role in reducing the microstrain (increased crystallinity) which was confirmed from the synchrotron X-ray diffraction studies. The sacrificial GO underlayer may contribute to relaxing the inhomogeneous internal strain of the α-Fe<SUB>2</SUB>O<SUB>3</SUB> nanorods and reducing the deformation of FTO to an extent. In other words, the reduction of the microstrain minimizes the lattice imperfections and defects at the FTO/α-Fe<SUB>2</SUB>O<SUB>3</SUB> interface, which may enhance the charge collection efficiency, as demonstrated by the impedance measurements. From the EXAFS analysis, it is clearly evident that the sacrificial GO underlayer does not affect the structure of α-Fe<SUB>2</SUB>O<SUB>3</SUB> in the short range. The effects of the GO sacrificial layers are restricted to the FTO/α-Fe<SUB>2</SUB>O<SUB>3</SUB> interface, and they do not affect the bulk properties of α-Fe<SUB>2</SUB>O<SUB>3</SUB>.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2015/jpccck.2015.119.issue-34/acs.jpcc.5b06450/production/images/medium/jp-2015-06450j_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp5b06450'>ACS Electronic Supporting Info</A></P>

Fabrication of superior α-Fe₂O₃ nanorod photoanodes through ex-situ Sn-doping for solar water splitting

Annamalai, Alagappan,Shinde, Pravin S.,Jeon, Tae Hwa,Lee, Hyun Hwi,Kim, Hyun Gyu,Choi, Wonyong,Jang, Jum Suk Elsevier 2016 Solar energy materials and solar cells Vol.144 No.-

<P><B>Abstract</B></P> <P>Doping transition metals into 1-D nanostructures is of crucial importance for their application in photovoltaics and photoelectrochemical (PEC) systems; performance enhancements arise from both dopant incorporation and the 1-D nanostructures. Both in-situ and ex-situ doping methods have been demonstrated for 1-D hematite (α-Fe<SUB>2</SUB>O<SUB>3</SUB>) nanostructures, with tin (Sn) as the dopant, for photoelectrochemical water oxidation. In-situ Sn-doped hematite photoanodes adopted a morphology consisting of nanocorals with the (104) plane as the preferred direction of crystal growth. As an alternative solution, ex-situ doping not only preserves the vertically-aligned nanorod morphology but also sustains the preferred orientation of the (110) axis, which is favorable for high conductivity in pristine hematite photoanodes. In-situ Sn-doping was carried out by the same method: Sn precursors were added and dissolved in ethanol during the hydrothermal synthesis. Ex-situ doping was carried out in two stages (during pre-deposition and during high temperature sintering). During pre-deposition, a defined amount of the Sn precursor was introduced near the surface region of the 1-D nanostructure, and the Sn content was controlled by changing the concentration of the precursor solution. In subsequent high temperature sintering (800°C), the dopant atoms diffused into the hematite lattice to attain the desired doping profile. We found that ex-situ Sn-doping resulted in a 60% increase in the photocurrent while in-situ Sn-doping yielded an increase of only 20% in the photocurrent, as compared with pristine hematite photoanodes, at 1.4 V <I>vs.</I> RHE. The improvement in the photocurrent was caused by a combination of Sn dopants in the hematite, which act as electron donors by increasing the donor density, and better surface charge transfer kinetics, thereby enhancing the overall device performance.</P> <P><B>Highlights</B></P> <P> <UL> <LI> In-situ and ex-situ doping methods were compared for α-Fe<SUB>2</SUB>O<SUB>3</SUB> with Sn as a dopant. </LI> <LI> Ex-situ Sn-doping has 60% higher photocurrent compared to pristine photoanodes. </LI> <LI> Ex-Situ retains highly conductive (110) crystal plane and nanorod morphology. </LI> <LI> In-situ has lot of grain boundaries (nano-corals) and less conductive (104) phase. </LI> <LI> All doping problems related to β-FeOOH phase has been avoided in ex-situ doping. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Application of Zn2SnO4 Nanoparticles in Dye Sensitized Solar Cells

( Alagappan Annamalai ),장점석,이만종 한국공업화학회 2014 한국공업화학회 연구논문 초록집 Vol.2014 No.0

Trade-off between Zr passivation and Sn doping on hematite nanorod photoanodes for efficient solar water oxidation: Effects of ZrO2 underlayer and FTO deformation

( Subramanian Arunprabaharan ),( Alagappan Annamalai ),이현휘,최선희,류정호,박정희,장점석 한국공업화학회 2016 한국공업화학회 연구논문 초록집 Vol.2016 No.0

Herein we report the influence of a ZrO2 underlayer on the PEC behavior of hematite nanorod photoanodes for efficient solar water splitting. Akaganite (β-FeOOH) nanorods were grown on FTO substrates treated with a ZrO2 underlayer. Sintering at 800°C transformed akaganite to the hematite (α-Fe2O3) phase and induced Sn diffusion into the crystal structure of hematite nanorods from the FTO substrate and surface migration of Zr atoms from the ZrO2 underlayer toward the top layer. A cathodic shift in the onset potential as well as photocurrent enhancement was achieved by surface passivation from the ZrO2 underlayer and Sn doping from the FTO substrate to the crystal lattice of hematite nanorods.

Cathodic shift in onset potential of hematite photoanodes by ZrO2 underlayer for efficient water splitting

( Subramanian Arunprabaharan ),( Alagappan Annamalai ),( Jum Suk Jang ) 한국공업화학회 2015 한국공업화학회 연구논문 초록집 Vol.2015 No.0

Herein we report that influence of Zr underlayer to hematite photoanodes for cathodic shift in onset potential as well as photocurrent. Akaganite (β-FeOOH) nanorods were grown on Zr underlayer treated FTO substrates. Sintering at 800°C transforms the akaganite to hematite phase and induces the Sn atoms from the FTO substrates will penetrate in to the hematite lattices along with Zr atoms. Zr underlayer treated sample shows the better performance of water splitting compared to the pristine photoanode. Cathodic shift in onset potential as well as photocurrent was done by penetration of Zr atoms from the underlayer to hematite lattices. 0.96 mA/cm2 at 1.23 VRHE was achieved for Zr based hematite photoanode with low turn on voltage 0.7 V vs. RHE. The crystalline structure and morphology for the Zr modified hematite photoanode almost identical to the pristine photoanode. Presence of Zirconium and Sn diffusion from the FTO were confirmed by XPS analysis. Electrochemical Impedance Spectroscopy reveals that the presence of Zr underlayer increases the charge transfer resistance along the various interfaces. The presence of ZrO2 underlayer decreases the FTO deformation and the sheet resistance of FTO is decreased. Mott Schottky analysis reveals that the donor density and flat band potential of the pristine and Zr underlayer treated photoanodes shows similar values. This is the first report on Zr underlayer based hematite photoanodes for the increment in photocurrent and cathodic shift in onset potential.

Trade-off between Zr Passivation and Sn Doping on Hematite Nanorod Photoanodes for Efficient Solar Water Oxidation: Effects of a ZrO₂ Underlayer and FTO Deformation

Subramanian, Arunprabaharan,Annamalai, Alagappan,Lee, Hyun Hwi,Choi, Sun Hee,Ryu, Jungho,Park, Jung Hee,Jang, Jum Suk American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.30

<P>Herein we report the influence of a ZrO2 underlayer on the PEC (photoelectrochemical) behavior of hematite nanorod photoanodes for efficient solar water splitting. Particular attention was given to the cathodic shift in onset potential and photocurrent enhancement. Akaganite (beta-FeOOH) nanorods were grown on ZrO2-coated FTO (fluorine-doped tin oxide) substrates. Sintering at 800 degrees C transformed akaganite to the hematite (alpha-Fe2O3) phase and induced Sn diffusion into the crystal structure of hematite nanorods from the FTO substrates and surface migration, shallow doping of Zr atoms from the ZrO2 underlayer. The ZrO2 underlayer-treated photoanode showed better water oxidation performance compared to the pristine (alpha-Fe2O3) photoanode. A cathodic shift in the onset potential and photocurrent enhancement was achieved by surface passivation and shallow doping of Zr from the ZrO2 underlayer, along with Sn doping from the FTO substrate to the crystal lattice of hematite nanorods. The Zr based hematite nanorod photoanode achieved 1 mA/cm(2) at 1.23 V-RHE with a low turn-on voltage of 0.80 V-RHE. Sn doping and Zr passivation, as well as shallow doping, were confirmed by XPS, I-ph, and M-S plot analyses. Electrochemical impedance spectroscopy revealed that the presence of a ZrO2 underlayer decreased the deformation of FTO substrate, improved electron transfer at the hematite/FTO interface and increased charge-transfer resistance at the electrolyte/hematite interface. This is the first systematic investigation of the effects of Zr passivation, shallow doping, and Sn doping on hematite nanorod photoanodes through application of a ZrO2 underlayer on the FTO substrate.</P>

Fine-tuning pulse-reverse electrodeposition for enhanced photoelectrochemical water oxidation performance of α-Fe₂O₃ photoanodes

( Pravin Shinde ),( Alagappan Annamalai ),( Jae Young Kim ),( Jae Sung Lee ),( Jum Suk Jang ) 한국공업화학회 2014 한국공업화학회 연구논문 초록집 Vol.2014 No.1

High-quality hematite (α-Fe₂O₃) photoanodes were synthesized from a sulfate electrolyte bath by pulse-reverse electrodeposition method. The influence of parameters of pulse electrodeposition process was systematically investigated such as duty cycle, pulse period and deposition time, on the structural, optical, morphological, and photoelectrochemical properties of the films. The optimized parameters of pulse duty cycle, pulse period and the deposition time were 20%, 10 ms and 45 s, respectively. The nanocrystalline morphology of the α-Fe₂O₃ was found to alter according to the process parameters. The α-Fe₂O₃ electrodes prepared by annealing at 550°C for 4h followed by 800°C for 15 min exhibited an optimum photocurrent density of 504 μA cm^-2 measured at 1.23 V vs. RHE.

Fine-Tuning Pulse Reverse Electrodeposition for Enhanced Photoelectrochemical Water Oxidation Performance of α-Fe₂O₃ Photoanodes

Shinde, Pravin S.,Annamalai, Alagappan,Kim, Jae Young,Choi, Sun Hee,Lee, Jae Sung,Jang, Jum Suk American Chemical Society 2015 The Journal of Physical Chemistry Part C Vol.119 No.10

<P>High:quality hematite (alpha-Fe2O3) photoanodes were syritheSized from a sulfate electrolyte bath by the pulse reverse electrodeposition (PRED) method. The influence of PRED parameters (viz, duty cycle, pulse period, and deposition time) was systematically investigated on the structural, optical, morphological, and photo electrochemical properties of the films. The optimized parameters of pulse duty cycle, pulse:period, and the deposition time were 20%, 10 ms, and 45 s, respectively. The granular and compact nanocrystalline morphology of the alpha-Fe2O3 was found to alter according to the process parameters. The alpha-Fe2O3 electrodes (film thickness similar to 200 nm) prepared by annealing at 550 degrees C for 4 h followed by 800 degrees C for 15 min exhibited an optimum photocurrent density of 504 mu A cm(-2) measured at 1.23 V vs RHE in 1 M NaOH electrolyte under 100 mW cm(-2) light illumination.</P>

Effect of tetravalent dopants on hematite nanostructure for enhanced photoelectrochemical water splitting

Subramanian, Arunprabaharan,Gracia-Espino, Eduardo,Annamalai, Alagappan,Lee, Hyun Hwi,Lee, Su Yong,Choi, Sun Hee,Jang, Jum Suk Elsevier 2018 APPLIED SURFACE SCIENCE - Vol.427 No.2

<P><B>Abstract</B></P> <P>In this paper, the influence of tetravalent dopants such as Si<SUP>4+</SUP>, Sn<SUP>4+</SUP>, Ti<SUP>4+</SUP>, and Zr<SUP>4+</SUP> on the hematite (α-Fe<SUB>2</SUB>O<SUB>3</SUB>) nanostructure for enhanced photoelectrochemical (PEC) water splitting are reported. The tetravalent doping was performed on hydrothermally grown akaganeite (β-FeOOH) nanorods on FTO (fluorine-doped tin-oxide) substrates via a simple dipping method for which the respective metal-precursor solution was used, followed by a high-temperature (800°C) sintering in a box furnace. The photocurrent density for the pristine (hematite) photoanode is ∼0.81mA/cm<SUP>2</SUP> at 1.23V<SUB>RHE</SUB>, with an onset potential of 0.72V<SUB>RHE</SUB>; however, the tetravalent dopants on the hematite nanostructures alter the properties of the pristine photoanode. The Si<SUP>4+</SUP>-doped hematite photoanode showed a slight photocurrent increment without a changing of the onset potential of the pristine photoanode. The Sn<SUP>4+</SUP>- and Ti<SUP>4+</SUP>-doped hematite photoanodes, however, showed an anodic shift of the onset potential with the photocurrent increment at a higher applied potential. Interestingly, the Zr<SUP>4+</SUP>-doped hematite photoanode exhibited an onset potential that is similar to those of the pristine and Si<SUP>4+</SUP>-doped hematite, but a larger photocurrent density that is similar to those of the Sn<SUP>4+</SUP>- and Ti<SUP>4+</SUP>-doped photoanodes was recorded. The photoactivity of the doped photoanodes at 1.23V<SUB>RHE</SUB> follows the order Zr > Sn > Ti > Si. The onset-potential shifts of the doped photoanodes were investigated using the <I>Ab initio</I> calculations that are well correlated with the experimental data. X-ray diffraction (XRD) and scanning-electron microscopy (FESEM) revealed that both the crystalline phase of the hematite and the nanorod morphology were preserved after the doping procedure. X-ray photoelectron spectroscopy (XPS) confirmed the presence of the tetravalent dopants on the hematite nanostructure. The charge-transfer resistance at the various interfaces of the doped photoanodes was studied using impedance spectroscopy. The doping on the hematite photoanodes was confirmed using the Mott-Schottky (MS) analysis.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Tetravalent dopants on hematite photoanodes for photoelectrochemical (PEC) water oxidation were studied. </LI> <LI> High temperature calcination (800°C/10min) transformed akaganeite to hematite phase and caused doping. </LI> <LI> Onset potential shift of the photoanodes were studied by experimental and computational methods. </LI> <LI> Zr doped hematite exhibited photocurrent density of 1.35mA/cm<SUP>2</SUP> at 1.23V<SUB>RHE</SUB> with lower anodic shift. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>Photoelectrochemical water oxidation performance of doped hematite photoanodes in 1 M NaOH solution at 1.23V<SUB>RHE</SUB> under 1 sun illumination.</P> <P>[DISPLAY OMISSION]</P>